1. Technical Field of the Invention
The present invention relates to radiometers or other devices which can detect light or other radiation and, more particularly, to a device which uses micro-electro-mechanical structure/system (MEMS) technology for the purpose of measuring radiation from, for example, a high intensity light source.
2. Description of Related Art
Every schoolchild is familiar with the light-mill type of radiometer (also known as William Crookes' Radiometer or a solar engine). This device typically consists of four vanes each of which is blackened on one side and silvered on the other. These vanes are attached to the arms of a rotor which is balanced on a vertical support is such a way that the rotor can turn with very little friction. The mechanism is then encased inside a clear glass bulb which has been pumped out to a high, but not perfect, vacuum.
When sunlight falls on the light-mill, the vanes turn in a direction whereby the blackened side appears to be pushed away from the source of the light. This would appear to be counterintuitive as one would expect light to be absorbed by the blackened side and reflected by the silver side thus causing movement whereby the silver side is pushed away from the light source. A number of explanations have been proposed for how the light-mill operates and why it rotates away from the blackened side. Most explanations, over the years, have been shown to incorrect. One currently accepted explanation is based on the concept of thermal transpiration which refers to the flow of gas through a porous plate caused by temperature differences between the two sides of the plate. Although the vanes of a light-mill are not porous, the concept of thermal transpiration is nonetheless pertinent to device operation. If the gas is initially at the same temperature on each side of the plate, and one side is then heated, there would be a flow of gas from the colder to the hotter side resulting in a higher pressure on the hotter side. With the non-porous vanes of the light-mill, one must focus attention instead on the edges of the vanes wherein the thermomolecular forces that are responsible for thermal transpiration further operate, with an effect known as thermal creep, to cause gas molecules from the blackened side to strike the side edges of the vanes and impart a higher force on the vanes than the colder molecules on the silver side. The resulting behavior is just as if there were a greater force being exerted on the blackened side of the vane.
But what about radiation received on a reflective surface, can it cause movement away from that reflective surface? The fact that electromagnetic radiation exerts a pressure upon any surface exposed to it was deduced theoretically by Maxwell, and proven experimentally by Lebedev, Nichols and Hull. The exerted pressure is very feeble, but can be detected by allowing the radiation to fall upon a delicately poised vane of reflective metal (also known as a Nichols radiometer). The Nichols radiometer measured radiation pressure by directing a beam of light selectively onto one or the other of a pair of small silvered glass mirrors suspended in the manner of a torsion balance by a fine quartz fiber within an enclosure in which the air pressure could be regulated. The light received and reflected by one vane was shown to upset the balance. Current experimentation with electromagnetic radiation exerted pressure relates to the use of solar sails which show promise for the use of solar radiation, when reflected by a large sail-like structure in the vacuum of space, as a drive source spacecraft propulsion.
It is also known in the art to use laser light for the purpose of transporting, suspending or trapping non-atomic sized particles in free space. A variety of micro-sized particles, including solids, solid dielectrics, semiconductors, liquids, aerosols and living cells have been shown in U.S. Pat. No. 6,636,676, the disclosure of which is hereby incorporated by reference, to be position and movement controllable within a hollow-core fiber using a laser light source.
The foregoing show that radiation, and in particular high intensity light, can cause movement of physical matter.
A number of micromechanical sensors are known in the art for use in measuring nuclear and electromagnetic radiation (see, for example, U.S. Pat. Nos. 5,977,544 and 6,118,124, the disclosures of which are hereby incorporated by reference). Such sensors typically utilize a MEMS-type micro-cantilever which is fabricated from materials that respond to impinging radiation by deflecting or changing the resonance frequency of the cantilever. These resulting physical changes (deflections or resonance) in the cantilever beam are then measured using any one of a number of known processes.
It is also known in the art to utilize semiconductor photocells, photodiodes and chemical photocells (like cadmium sulfide) to measure the intensity of light. These devices are not always acceptable because the semiconductor materials used in their construction can be damaged by high intensity radiation, there are certain unacceptable minimum size requirements for the construction of a reasonably accurate photometric element, and the devices tend to deteriorate with respect to accuracy of measurement as they age.